Alzheimers disease is associated with the abnormal deposits of amyloid beta protein (Ab) which occurs as fibrils within the cerebral neuropil. To characterize the way in which these fibrils assemble we have performed in vitro structural studies on synthetic peptides including the full-length Ab(1-40) as well as the Ab(10-35) which both contain the motif KLVFFAE associated with beta-sheet formation. We have used mass mapping in the scanning transmission electron microscope to determine the packing of beta-sheets within fibrils formed from these peptides under different pH conditions. Measurements of Ab(10-35) fibrils grown at pH 3.7 reveal protofilaments that contain just two stacked beta-sheets. At pH 7.4, Ab(10-35) forms a heterogeneous population of fibrils with a helically twisted structure. Measurements of fibrils grown from the naturally occurring full-length Ab(1-40) peptide at pH 7.4 reveal a different type of helically twisted protofilament containing three beta-sheets stacked together as a lamina. Our results demonstrate how the structure of fibrils grown from Alzheimer-related Ab peptides depends on the peptide length and on the growth conditions. - Alzheimer's disease, amyloid beta protein, scanning transmission electron microscopy, mass mapping